I used 1" sq thinwall for the longitudinal upper mounts, and 5/8 (IIRC) emt to connect it all together. Rather than welding it together, I brazed it: much stronger because it's a low temp process. Be carefull welding/brazing emt. The cadmium plating releases nasty gasses.
Here is the frame test fit my my killer:

I used plastic cutting board top and givi-case hookup hardware. Finished prod looks like this;

Nice home built rack!
I'm in the process of getting my 250 sorted for some light touring/ commuting with a set-up that looks a lot like Brash1's. My worry with my/his arrangement is that the turn signals are largely obscured. Looks like you have a pretty nice solution. Could you provide some more pic/ info?
Cheers!